internal/fieldnum: generate field numbers for the google.protobuf package

Generate field numbers for the well-known types,
so that encoding/jsonpb can benefit from them as well.

This CL fixes internal/cmd/generate-protos, which was silently failing
because the modulePath was not properly initialized. We fix this by
moving it to the start of the init function.

Change-Id: I87637176f29218cffa512b4baa49f39dae924061
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/168497
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/internal/fieldnum/api_gen.go b/internal/fieldnum/api_gen.go
new file mode 100644
index 0000000..6014c86
--- /dev/null
+++ b/internal/fieldnum/api_gen.go
@@ -0,0 +1,35 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style.
+// license that can be found in the LICENSE file.
+
+// Code generated by generate-protos. DO NOT EDIT.
+
+package fieldnum
+
+// Field numbers for google.protobuf.Api.
+const (
+	Api_Name          = 1 // optional string
+	Api_Methods       = 2 // repeated google.protobuf.Method
+	Api_Options       = 3 // repeated google.protobuf.Option
+	Api_Version       = 4 // optional string
+	Api_SourceContext = 5 // optional google.protobuf.SourceContext
+	Api_Mixins        = 6 // repeated google.protobuf.Mixin
+	Api_Syntax        = 7 // optional google.protobuf.Syntax
+)
+
+// Field numbers for google.protobuf.Method.
+const (
+	Method_Name              = 1 // optional string
+	Method_RequestTypeUrl    = 2 // optional string
+	Method_RequestStreaming  = 3 // optional bool
+	Method_ResponseTypeUrl   = 4 // optional string
+	Method_ResponseStreaming = 5 // optional bool
+	Method_Options           = 6 // repeated google.protobuf.Option
+	Method_Syntax            = 7 // optional google.protobuf.Syntax
+)
+
+// Field numbers for google.protobuf.Mixin.
+const (
+	Mixin_Name = 1 // optional string
+	Mixin_Root = 2 // optional string
+)